The basic principle of using time-of-flight (TOF) measurements for range finding applications is to measure how long it takes for radiation, e.g. photons to travel over an unknown distance. The unknown distance can then be deduced from the measured time of flight in combination with the known speed of the radiation such as light.
Many ways of how to modulate a light source for such TOF measurements, and which strategy to follow for making the distance measurement are known to a person skilled in the art and are described in patents and scientific literature. Most of these range-finding systems use a receiver in which a mixer is used to demodulate an incoming photocurrent for finding e.g. a phase or a time period for distance estimation. The photocurrent is typically mixed with a reference signal.
There are basically two ways to achieve this mixing. A first way is to use transistors in a multiplier configuration, for mixing the photocurrent signal with the reference signal. WO 2004/012269 describes a readout circuit using this technique. A light source is pulsed to illuminate a scene, for example comprising one or more objects, and the phase difference between the light reflected from the scene and the original phase of the light source is measured. In order to measure the phase difference, a CMOS photosensor may be used to receive the reflected light and store charge generated during different portions of time in different storage nodes or pixel cells. The difference between the amount of charge stored in different storage nodes can be used to determine the phase difference between the original light illuminating the scene and the light reflected from the scene. This phase difference can in turn be used to determine the distance to the scene. This and other transistor mixing methods will be called “transistor-mixing-methods”.
A second way of achieving the mixing is by redirecting photo-generated minority carriers in the substrate, before they are detected by a diode-junction or by a potential well. WO 98/10255 and WO 99/60629 show such methods and corresponding devices for determining the phase and/or amplitude of incident modulated light. By applying a reference modulation voltage over two photo-gates, the generated minority carriers in the substrate arrive preferentially at one of two detector accumulation zones. In WO 98/10255, these accumulation zones are potential wells, created by a voltage on an adjacent accumulation gate. In WO 99/60629, these accumulation zones are pn-junctions. In EP-03077744.5 co-pending herewith, a bipolar alternative and enhanced version is described using a reference majority current for redirecting the photo-generated carriers in the substrate towards detecting junctions. These second ways of mixing before diode junction detection will be called “substrate-mixing-methods”.
Depending on the target specifications, a substrate-mixing-method or a transistor-mixing-method will be preferred.
With both mixing methods it remains a problem to separate signals originating from background light efficiently from signals originating from TOF-light. The background light that is present on an area in a scene of which the distance is to be measured, can be six orders of magnitude larger than the light present on this same area and originating from the modulated light source. It is known from literature to reduce this large difference to some extent by using an optical filter, which attenuates the visible background light from the TOF light based on wavelength differences. In this way a reduction of an order of magnitude can be obtained. With a narrow-band optical pass filter and using a narrow-band laser light source, possibly two orders of magnitude can be overcome. However, LED light sources are preferred light sources for future TOF range finders, since they may emit Watts of light, whereas lasers may only emit milli-Watts of light in free space for eye-safety reasons.
Further it is difficult to make low-pass filters with a −3 dB frequency in the 10 Hz-10 kHz range for range finding systems on a small silicon circuit area, such that each camera pixel can have its own averaging filter for averaging out the noise after the mixer.
Therefore, no readily usable solution is known for separating signals originating from background light from signals originating from TOF-light by means of a small circuit while not deteriorating the signal to noise ratio and still achieving 3D camera operation.